Certain optical fibers have been developed having a core made of only pure silica and fluorine, such as those fibers described in U.S. Pat. No. 6,853,798, incorporated herein by reference in its entirety. The cores may be produced by chemical vapor deposition. Fibers with a silica core, doped predominantly with fluorine, can allow hydrogen to enter the core freely. This behavior is illustrated by, for example, the induced loss at 1380 nm (“water peak”) depicted in FIG. 1, which is understood to be due to a reaction between H2 and silica defects to create OH-ions, resulting in a permanent loss. In contrast, much of the loss at 1241 nm (“hydrogen peak”) is transient, and is understood to be due to unreacted H2.
Various techniques have been used to address the issue of high losses, e.g., at 1380 nm and 1241 nm. For example, U.S. Pat. No. 8,542,967, incorporated herein by reference in its entirety, teaches introducing 0.1 wt % to 5 wt % germanium to its core, as the integration of germanium into the silica and fluorine matrix may prevent hydrogen from completely flooding the core. FIG. 2 contrasts the induced loss of an optical fiber with 0% germanium in the core with one containing 3 mol % germanium in the core. At 1380 nm and at 1241 nm, the induced loss of the germanium doped fiber is lower than that of the 0% germanium doped fiber.
However, while the presence of germanium in the core may mitigate some of the losses at 1380 nm and at 1241 nm, the presence of germanium can create new problems. U.S. Pat. No. 6,853,798 specifically identifies that the presence of germanium promotes the formation of OH bonds in a downhole environment. Further, germanium-induced defects, combined with the presence of hydrogen, can cause different peaks to form. These peaks, which tend to form at wavelengths less than 800 nm, often increase over time, with peaks becoming large enough to negatively impact wavelengths used by typical downhole geophysical and geothermal well sensors.
A need exists, therefore, to address issues associated with hydrogen getting into a core of pure silica and/or fluorine, without relying on the use of materials such as germanium, phosphorus, and titanium in the core that can introduce additional undesirable features.